Analytical Technology, Inc. (ATI) recently introduced an dissolved-oxygen monitor using an optical sensor in lieu of the conventional electrolyte-and-membrane system which they've offered for many years. Both systems are available with the Auto-Clean system which ATI developed to provide consistent, reliable cleaning of the sensor head, since DO monitors are usually installed in heavy-fouling applications. In a four-month test, both types of sensors have proven themselves to be highly reliable and almost identical in performance. This means that ATI can deliver an effective optical DO sensor for those who want one, but also that those who consider the optical sensor an "upgrade" should be aware that they might not need to spend extra to get high performance.